Generally in an automobile a battery 1 and a generator 3 driven by an engine 2 are connected in parallel to an electric load 4, for example as shown in FIG. 1, and a generating voltage of the generator 3 is normally defined to be a predetermined voltage of the battery 1 (14.5 V for example) so that the voltage may be sufficient for charging of the battery 1 and for supplying the electric load 4. The generating power of the generator 3 changes according to a use condition of the lectric load, and the load to the engine which drives the generator 3 changes in proportion thereto. In the case that the electric load 4 is small and the engine 2 is in a condition other than deceleration, the load of the generator 3 to the engine 2 may be reduced by lowering the generating voltage of the generator 3 to a charging voltage of the battery 1 (12.5 V for example), thereby reducing the load of the generator 3 and reducing a driving loss of the engine 2 to thereby improve fuel consumption.
FIG. 2 shows characteristics of an output current of the generator 3 relative to engine rotational speed (RPM), provided that the generating voltage of the generator is defined as a parameter. It is apparent from FIG. 2 that the higher the generating voltage, the greater the generated energy, and consequently the faster the battery is charged. With use of these characteristics, when power is generated by the lower voltage at a light electric load which is in a normal condition, a charging current is reduced to decrease the engine load. On the other hand, at a heavy electric load, the generating voltage is switched to a higher value (e.g. 14.5) to increase the generated energy, thereby sufficiently meeting load requirements and suppressing discharging of the battery.
Conventionally, in a circuit including the battery and the generator driven by the engine which ar connected with each other in parallel to the electric load, the battery voltage is detected and compared with a predetermined reference voltage so as to maintain the battery voltage at a constant value according to a charged condition of the battery, a closed condition of the electric load, and an operating condition of the engine. Then, according to the comparative result, a reference generating voltage of a voltage regulator for On-Off control of the field current of the generator is controlled to be switched to a high or low value.
However, in the above-mentioned conventional control of the engine driven generator, when the generator is changed to a reduced generating position of lower generated energy, and the electric load, such a head lamp and/or an air conditioner generating a large rush of current upon application of power source, is applied, and at this time the generator is switched to a large generating position of higher generated energy, the "ON" time of the field current at a higher generating voltage in an initial stage of the switching is increased and the generated energy of the generator is rapidly increased for a period of time until a balanced condition is reached nd this causes a rapid decrease in the engine rotational speed, thereby impairing performance of the engine. Stated differently, if the generator is generating at low power (e.g. at 12.5 V) and a load (e.g. headlights or air conditioner) is switched on, the generator (via control of its field) is caused to generate at a higher power level (e.g. 14.5 V). The generated power sharply increases thereby causing the engine RPM (Ne) to drop.
A further understanding of background systems may be found in Goto U.S. patent application Ser. No. 616,262, filed May 31, 1984 and assigned to the assignee of this application, and U.S. Pat. Nos. 3,491,285, 3,659,188 and 4,300,088.